LayoutInflater 详解

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一.怎么使用

1.1获取到LayoutInflater的实例，有两种方法可以获取到

1.1.1第一种(Android给我们做了一下封装）

LayoutInflater layoutInflater = LayoutInflater.from(context);

1.1.2 第二种（这个不太常用）

LayoutInflater layoutInflater = (LayoutInflater) context        .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

1.2 调用它的inflate()方法来加载布局

inflate()方法一般接收两个参数，第一个参数就是要加载的布局id，第二个参数是指给该布局的外部再嵌套一层父布局，如果不需要就直接传null

    layoutInflater.inflate(resourceId, root);

或者用三个参数（其实上面的函数也是调用了这个函数）

inflate(int resource, ViewGroup root, boolean attachToRoot)

1.3使用

1.3.1activity布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/main_layout"    android:layout_width="match_parent"    android:layout_height="match_parent" ></LinearLayout>

1.3.2 button控件布局

<Button xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:text="Button" ></Button>

1.3.3 Activity 通过LayoutInflater来将button_layout这个布局添加到主布局文件的LinearLayout中

public class MainActivity extends Activity {    private LinearLayout mainLayout;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mainLayout = (LinearLayout) findViewById(R.id.main_layout);        LayoutInflater layoutInflater = LayoutInflater.from(this);        View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);        mainLayout.addView(buttonLayout);    }}

1.3.4 在Fragment中使用有返回一个view 不需要用addView了

public class CareFragment extends Fragment {    @Nullable    @Override    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {        View view = inflater.inflate(R.layout.fragment_care, container, false);        mToolbar = (MyToolBar) view.findViewById(R.id.mToobarcare);        return view;    }

二.详细分析

2.1 这里是源码一调用了源码二

源码inflate(两个参数)

    public View inflate(int resource, ViewGroup root) {        return inflate(resource, root, root != null);    }

2.2 这里是源码二调用了源码三

源码二 inflate(三个参数)

如果root为null，attachToRoot将失去作用，设置任何值都没有意义。
如果root不为null，attachToRoot设为true，则会给加载的布局文件的指定一个父布局，即root。
如果root不为null，attachToRoot设为false，则会将布局文件最外层的所有layout属性进行设置，当该view被添加到父view当中时，这些layout属性会自动生效。
在不设置attachToRoot参数的情况下，如果root不为null，attachToRoot参数默认为true。

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {        final Resources res = getContext().getResources();        if (DEBUG) {            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("                    + Integer.toHexString(resource) + ")");        }        final XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

2.3这里是源码三调用了源码四（前几个其实最后都调用了源码三）

源码三 inflate(三个参数)

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context)mConstructorArgs[0];            mConstructorArgs[0] = mContext;            View result = root;            try {                // Look for the root node.                int type;                while ((type = parser.next()) != XmlPullParser.START_TAG &&                        type != XmlPullParser.END_DOCUMENT) {                    // Empty                }                if (type != XmlPullParser.START_TAG) {                    throw new InflateException(parser.getPositionDescription()                            + ": No start tag found!");                }                final String name = parser.getName();                if (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }                if (TAG_MERGE.equals(name)) {                    if (root == null || !attachToRoot) {                        throw new InflateException("<merge /> can be used only with a valid "                                + "ViewGroup root and attachToRoot=true");                    }                    rInflate(parser, root, attrs, false, false);                } else {                    // Temp is the root view that was found in the xml                    final View temp = createViewFromTag(root, name, attrs, false);                    ViewGroup.LayoutParams params = null;                    if (root != null) {                        if (DEBUG) {                            System.out.println("Creating params from root: " +                                    root);                        }                        // Create layout params that match root, if supplied                        params = root.generateLayoutParams(attrs);                        if (!attachToRoot) {                            // Set the layout params for temp if we are not                            // attaching. (If we are, we use addView, below)                            temp.setLayoutParams(params);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }                    // Inflate all children under temp                    rInflate(parser, temp, attrs, true, true);                    if (DEBUG) {                        System.out.println("-----> done inflating children");                    }                    // We are supposed to attach all the views we found (int temp)                    // to root. Do that now.                    if (root != null && attachToRoot) {                        root.addView(temp, params);                    }                    // Decide whether to return the root that was passed in or the                    // top view found in xml.                    if (root == null || !attachToRoot) {                        result = temp;                    }                }            } catch (XmlPullParserException e) {                InflateException ex = new InflateException(e.getMessage());                ex.initCause(e);                throw ex;            } catch (IOException e) {                InflateException ex = new InflateException(                        parser.getPositionDescription()                        + ": " + e.getMessage());                ex.initCause(e);                throw ex;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;            }            Trace.traceEnd(Trace.TRACE_TAG_VIEW);            return result;        }    }

LayoutInflater其实就是使用Android提供的pull解析方式来解析布局文件的,这里我们注意看下第41行，调用了createViewFromTag()这个方法，并把节点名和参数传了进去。它是用于根据节点名来创建View对象的。在createViewFromTag()方法的内部又会去调用createView()方法，然后使用反射的方式创建出View的实例并返回。63行会调用 rInflate()

2.4 源码三调用了源码四

源码四 rInflate(三个参数)

private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)        throws XmlPullParserException, IOException {    final int depth = parser.getDepth();    int type;    while (((type = parser.next()) != XmlPullParser.END_TAG ||            parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {        if (type != XmlPullParser.START_TAG) {            continue;        }        final String name = parser.getName();        if (TAG_REQUEST_FOCUS.equals(name)) {            parseRequestFocus(parser, parent);        } else if (TAG_INCLUDE.equals(name)) {            if (parser.getDepth() == 0) {                throw new InflateException("<include /> cannot be the root element");            }            parseInclude(parser, parent, attrs);        } else if (TAG_MERGE.equals(name)) {            throw new InflateException("<merge /> must be the root element");        } else {            final View view = createViewFromTag(name, attrs);            final ViewGroup viewGroup = (ViewGroup) parent;            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);            rInflate(parser, view, attrs);            viewGroup.addView(view, params);        }    }    parent.onFinishInflate();}

在第21行同样是createViewFromTag()方法来创建View的实例，然后还会在第24行递归调用rInflate()方法来查找这个View下的子元素，每次递归完成后则将这个View添加到父布局当中。
这样的话，把整个布局文件都解析完成后就形成了一个完整的DOM结构，最终会把最顶层的根布局返回，至此inflate()过程全部结束。

2.5 另外layout_width等等属性必须要在外层有个父布局

Activity在setContentView()方法中，Android会自动在布局文件的最外层再嵌套一个FrameLayout

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